got the message from tracy So hope this helps when you go to the app from the menu click on the clan wars tab ,,once you are the clan wars page in the upper left corner is the menu tab hit that tab and if you have received the diamond division invite that is where it should be. The clan leader can opt in when the clan leader hits the invite it should pull up a page with the time slots pick a time slot and opt in,,,, the opt out is also on that invite opt in is lower right corner opt out lower left corner. You want to opt in as soon as possible to make it for this clan war.

I had a similar issue when I was trying to verify that my clan was indeed in the Diamond Division, I was able to contact @beachheadstudio via Twitter and they were able to help me. Beachhead Studios are the ones who organize Clan Wars. Good Luck.